1. Field of the Invention
This invention relates to the mooring of structures used in the exploration for and production of oil and gas at offshore locations, and particularly to an apparatus and method for mooring tension leg platforms.
2. Description of the Prior Art
One form of marine structure proposed for use in the exploration for and production of oil and gas at offshore locations especially in deep water, is the tension leg platform (TLP). The TLP comprises a working structure which is supported by its own buoyancy and which is drawn down to the desired working position by three or more tension cable legs connecting the working structure to a plurality of anchors on the ocean floor. The tensioned cables restrict the movement of the working structure and render the TLP relatively insensitive to the natural forces of wind and waves which would otherwise tend to displace and disturb the working structure.
Characteristics of the TLP which render it particularly suited for use in deep waters include: the time required for construction and placement of the TLP is considerably less than for other platform designs which are supported by steel or concrete columns from the ocean floor; since only longer cables and possibly heavier anchors are required to increase the operating depth of the TLP, the cost of the TLP is relatively insensitive to water depth as compared to the bottom-supported platforms; and the TLP is more readily salvagable at the end of an unsuccessful exploration program or at the completion of a production program.
One problem encountered when the TLP is used in deep waters is that tension cable fatigue can be accelerated by the fluttering induced by the flow of water past the tensioned cables. This effect is particularly serious when the flutter frequency is near a natural or resonant frequency of the cable. Resonant flutter induced by vortex shedding has been shown to be detrimental to marine cables (Vandiver et al., "A Field Study of Vortex-Excited Vibrations of Marine Cables", paper OTC 2491 presented at the Eighth Annual Offshore Technology Conference in Houston, Tex. in May 1976) to marine risers (U.S. Pat. No. 3,978,804 to Beynet et al.) and to marine pipelines (Mes, M. J., "Vortex Shedding Can Cause Pipe Lines to Break", Pipeline and Gas Journal, August, 1976). U.S. Pat. No. 3,978,804 discloses the use of a plurality of riser spacers, along the length of the riser legs of a vertically moored platform (VMP), to change the natural or resonant frequency of the individual risers above the flutter frequencies caused by the motion of the water past the risers. The riser spacers are rigid templates which support the risers of each platform leg against lateral movement in order to prevent collision between the risers and reduce the unsupported length of each riser. However, the individual legs of the platform are not interconnected and the risers of each leg are able to flutter as a group. In order to avoid this problem, the templates are designed so as to increase the drag effect of the overall system in order to dissipate this flutter energy. This increased drag, however, renders the platform more susceptible to severe natural forces.
In the drilling of offshore wells from a floating platform, it is necessary to use a riser, commonly known as a marine riser, which extends from the working deck of the platform to the subsea wellhead. The riser is in effect an elongated enclosure which surrounds and protects the drill string and other pipes and tools which are passed from the platform to the wellhead, or vice versa. During the drilling of the well, the riser also provides an enclosed pathway for the return of drilling fluids to the working deck. The riser is normally tensioned to prevent bending or buckling and to thereby reduce the friction between the drill string and the riser. For a tension leg platform moored with tensioned cables, the risers are suspended from the center portion of the platform, as compared to other types of moored platforms in which the risers are positioned at the perimeter of the platform and also serve as the platform legs.
The drilling from the VMP of U.S. Pat. No. 3,978,804 is done through the risers of the platform legs and because the risers are located at the perimeter of the platform, another problem associated with this platform is that the risers are subject to damage due to torsion of the platform caused by severe natural forces, which, during the drilling of wells, would cause the drillstring to scrape against the risers and damage them.
The problem of distortion of the riser during drilling operation from a tension leg platform is addressed by U.S. Pats. Nos. 3,996,755 and 3,983,706 to Kalinowski, which discloses the use of one or more adjustable riser bracing devices which serve to dynamically position the centralized riser to reduce the lateral distortion thereof caused by severe natural conditions. While perhaps stabilizing the riser, the adjustment of the braces during platform displacement due to natural forces significantly increases the tension on the leg cables thereby severely stressing the tensioned cables and resulting in either a draw-down of the floating structure or, alternatively, a lifting of the subsea anchors, both of which events are more significant problems during severe weather conditions than the distortion of the riser pipe.
Hence, a need exists for a simple but effective method for reducing the rate of fatigue of the tension cables and risers of a tension leg platform due to resonant flutter without adversely affecting other characteristics of the platform.
It is therefore a primary object of this invention to provide a method and apparatus for mooring a buoyant structure at offshore locations.
Another object of this invention is to provide a method and apparatus for mooring tension leg platforms at offshore locations such that the useful life of the tension legs is prolonged.
Still another object of this invention is to provide a simple but effective mooring system for a tension leg platform which eliminates resonant flutter of the tension legs and thereby substantially reduces cable fatigue.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description taken in conjunection with the accompanying drawings.